Cephalosporin antibiotics as new corrosion inhibitors for nickel in HCl solution

Cephalosporin antibiotics as new corrosion inhibitorsfor nickel in HCl solution

Abd El-Aziz S. Fouda • Mohamed M. Farahat •

Metwally Abdallah

Received: 13 November 2012 / Accepted: 8 January 2013 / Published online: 24 January 2013

� Springer Science+Business Media Dordrecht 2013

Abstract Inhibition of nickel corrosion in 1 M HCl solution in the absence and

presence of some Cephalosporin antibiotics derivatives was investigated using

potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and

electrochemical frequency modulation (EFM) techniques. The results obtained

show that the inhibition efficiency of these compounds depends on their concen-

trations and chemical structures. The inhibitive action of these compounds was

discussed in terms of blocking the electrode surface by adsorption of the molecules

through the active centers contained in their structures following the Langmuir

adsorption isotherm. The polarization measurement showed that these inhibitors are

acting as mixed inhibitors for both anodic and cathodic reactions. The effect of

temperature on the rate of corrosion in the absence and presence of these com-

pounds was also studied. The efficiencies obtained from the potentiodynamic

polarization technique were in good agreement with those obtained from EIS and

EFM techniques. This proves the validity of these tools in the measurements of the

investigated inhibitors.

Keywords Nickel � Corrosion inhibition � HCl � Electrochemical techniques �Cephalosporin antibiotics

A. E.-A. S. Fouda (&) � M. M. Farahat

Department of Chemistry, Faculty of Science, El-Mansoura University, El-Mansoura 35516, Egypt

e-mail: [email protected]

M. Abdallah

Department of Chemistry, Faculty of Applied Science, Um Al-Qura University,

Makkah, KSA

e-mail: [email protected]

123

Res Chem Intermed (2014) 40:1249–1266

DOI 10.1007/s11164-013-1036-0

Introduction

Nickel is used in many industrial processes because of its advantages, and in

consumer products, including stainless steel, magnets, coinage, rechargeable

batteries, electric guitar strings, and special alloys. It is also used for plating and

as a green tint in glass. Nickel is pre-eminently an alloy metal, and its chief use is in

nickel steels and nickel cast irons, of which there are many varieties. It is also

widely used in many other alloys, such as nickel brasses, bronzes, and alloys with

copper, chromium, aluminum, lead, cobalt, silver, and gold. Hydrochloric acid

solutions are used for pickling, and chemical and electrochemical etching of nickel

alloys. It is very important to add inhibitors to decrease the corrosion rate of nickel

in such solutions. Compounds with functional groups containing hetero-atoms,

which can donate lone pairs of electrons, are found to be particularly useful as

inhibitors for metal corrosion [1–6]. Also, organic substances containing polar

functions with nitrogen, oxygen, and or sulfur atoms in a conjugated system and

compounds with p-bonds have been reported to show good inhibiting properties

[7–12]. Both features obviously can be combined within the same molecule such

as drugs. Recently, the use of antibiotics and other drugs have been investigated

[13–18] and their inhibition efficiencies have been linked with their heterocyclic

nature. Ciprofloxacin was investigated [19] as a corrosion inhibitor for the corrosion

of mild steel in acidic medium. Also, the drug amoxycillin [20] was used as a

corrosion inhibitor for mild steel in 1 N hydrochloric acid solution. Generally, it has

been assumed that the first stage in the action mechanism of the inhibitors in

aggressive acid media is the adsorption of the inhibitors onto the metal surface. The

processes of adsorption of inhibitors are influenced by the nature and distribution of

charge in the molecule, the type of aggressive electrolyte, the type of interaction

between organic molecules, and the principal types of interaction between organic

inhibitors and the metal surface.

The investigated pharmaceutical compounds, which are used in the treatment of

hypertension diseases, are non-toxic, cheap, and environmentally friendly. They

contain reactive centers like N atoms and aromatic rings with delocalize p-electron

systems, which can aid their adsorption onto metal surfaces. Furthermore, they have

high molecular weights and are likely to effectively cover more surface area (due to

adsorption) of the metal, thus preventing corrosion from taking place.

The objective of the present investigation is to study the corrosion inhibition of

nickel in acidic medium using some cephalosporin antibiotics derivatives and to

propose a suitable mechanism for the inhibition using the potentiodynamic

polarization and ac impedance spectroscopy methods. The names, chemical and

molecular structures of the investigated compounds are shown in Table 1.

Experimental details

Materials and solutions

The chemical composition of nickel was 99.9 % BDH grade. For polarization

measurements, nickel electrodes were cut from Ni wire (diameter 0.5 mm). The

1250 A. E.-A. S. Fouda et al.

123

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Cephalosporin antibiotics as new corrosion inhibitor 1251

123

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etyl]

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123

electrodes were of 1 cm in length. The samples were embedded in a glass tube.

Epoxy resin was used to stick the sample to the glass tube. The electrode was

abraded with different grades of emery papers, degreased with acetone and rinsed

by bidistilled water. All chemicals and reagents used were of analytical grade.

Cephalosporin antibiotics were supplied by Egyptian Pharmaceutical Industries.

Stock solutions (1,000 ppm) of investigated compounds were prepared by

dissolving 1 g of each material in 1 L of bidistilled water. The measurements were

carried out at 25 and 40 �C using a thermostatic water bath controlled to ±1 �C.

Measurements

Polarization measurements

In this method, the working electrode was immersed in the test solution for 30 min

until the open potential circuit potential was reached. After that, the working

electrode was polarized in both cathodic and anodic directions. The values of

corrosion current density (icorr) were calculated from the extrapolation of Tafel lines

to the pre-determined open circuit potential. A standard ASTM glass electrochem-

ical cell was used. Platinum electrode was used as auxiliary electrode. All potentials

were measured against saturated calomel electrode (SCE) as a reference electrode.

Polarization measurements were carried from -1,200 to ?200 mV with respect to

corrosion potential (Ecorr) at a scanning rate of 1 mV s-1, and % IE was determined

as:

%IE ¼ 1� icorr=i�

corr

� �� 100 ð1Þ

where icorr and i8corr are the current densities in the absence and presence of

inhibitors, respectively.

Electrochemical impedance spectroscopy measurements (EIS)

Electrochemical impedance spectroscopy measurements were carried out at

25 ± 1 �C with the software program EIS 300. The measurements were carried

out using AC signal 10 mV peak to peak at the open circuit potential in the

frequency range of 100 kHz–0.5 Hz.

Electrochemical frequency modulation (EFM)

Electrochemical frequency modulation is a non-destructive corrosion measurement

technique that can directly give values of the corrosion current without prior

knowledge of Tafel constants. Like EIS, it is a small signal ac technique. Unlike

EIS, however, two sine waves (at different frequencies) are applied to the cell

simultaneously. Because current is a non-linear function of potential, the system

responds in a non-linear way to the potential excitation.

The current response contains not only the input frequencies but also the

frequency components which are the sum, difference, and multiples of the two input


123

frequencies. The two frequencies may not be chosen at random. They must both be

small, integer multiples of a base frequency that determines the length of the

experiment. Each spectrum is a current response as a function of frequency. The two

large peaks, with amplitudes of about 100 A, are the response to the 2- and 5-Hz

excitation frequencies. Those peaks between 1 and 20 A are the harmonics, sums,

and differences of the two excitation frequencies. These peaks are used by the

EFM140 software package to calculate the corrosion current and the Tafel

constants. It is important to note that between the peaks the current response is very

small. There is nearly no response (\100 nA) at 4.5 Hz, for example, while the

frequencies and amplitudes of the peaks are not coincidences, but are the direct

consequences of the EFM theory.

All electrochemical measurements were performed using a Gamry Instrument

Potentiostat/Galvanostat/ZRA. This includes a Gamry framework system based on

the ESA 400. Gamry applications include DC105 for corrosion measurements,

EIS300 software for EIS, and EFM140 software for EFM along with a computer for

collecting data. Echem Analyst 5.58 software was used for plotting, graphing, and

fitting data.

Results and discussion

Potentiodynamic polarization technique

Figure 1 shows typical anodic and cathodic Tafel polarization curves for nickel in

1 M HCl in the absence and presence of varying concentrations of compound 4 at

25 �C. Similar curves were obtained for the other compounds (not shown). As

reflected from the graph, the additive exhibits a significant effect on the corrosion

current density (icorr) and the corrosion potential (Ecorr) values. Table 2 shows the

effect of the inhibitor concentration on the corrosion kinetics parameters, such as

Tafel slopes (ba, bc), corrosion potential (Ecorr), corrosion current density (icorr), and

inhibition efficiency (% IE). The results of Table 1 indicate that the Tafel lines are

shifted to more negative and more positive potentials for the cathodic and the anodic

processes, respectively, relative to the uninhibited (blank) curve. This means that

these additives influence both the cathodic and the anodic processes, and that the

process of inhibition is believed to be a mixed inhibition process, i.e., the inhibitors

are of mixed type. It is also observed that the presence of these additives does not

shift Ecorr remarkably, and therefore these additives could be regarded as mixed-

type inhibitors and their inhibition occurred by blocking effect mechanism [21]. The

slopes of the cathodic and anodic Tafel lines are approximately constant and

independent of the inhibitor concentration. This behavior suggests that the inhibitor

molecules have no effect on the metal dissolution mechanism. A decrease in the

corrosion current density (icorr) was observed by increasing the concentration of the

inhibitor used. The order of % IE obtained from polarization measurements is as

follows: 1 [ 2 [ 3 [ 4.


123

Adsorption isotherm

The adsorption of the inhibitors is influenced by the nature and charge of the metal,

the chemical structure of the inhibitors, the distribution of the charge in the

molecule, and the type of electrolyte [22–24]. Important information about the

interaction between the inhibitor and Ni surface can be obtained from the adsorption

isotherm.

The values of surface coverage, h, increase with the inhibitor concentration, this

is attributed to more adsorption of inhibitors onto the Ni surface. The adsorption of

organic adsorbate on the surface of nickel electrode is regarded as a substitutional

adsorption process between the organic compound in the aqueous phase (Orgaq) and

the H2O molecules adsorbed on the nickel surface (H2O)ads [25].

Org solð Þ þ x ðH2OÞads ! Org adsð Þ þ x H2OðsolÞ ð2Þ

where x is the size ratio, that is, the number of H2O molecules replaced by one

organic molecule.

Attempts were made to fit h values to various isotherms including Frumkin,

Langmuir, Temkin, and Freundlich. The results were best fitted by far by the

Langmuir adsorption isotherm which has the following equation:

C=h ¼ 1=K þ C ð3Þ

where C is the inhibitor concentration in the electrolyte and K is the equilibrium

constant for the adsorption/desorption process. The value of K is related to the free

energy of adsorption, DG�ads, by the equation:

-1.0 -0.5 0.0 0.5-7

-6

-5

-4

-3

-2

-1

0

μ

Blank (1 M HCl) 10 PPM (4) 20 PPM (4) 30 PPM (4) 40 PPM (4) 50 PPM (4) 60 PPM (4)

Fig. 1 Potentiodynamic polarization curves for nickel in 1 M HCl in the absence and presence ofdifferent concentrations of compound 4 at 25 �C


123

K ¼ 1=55:5exp DG�ads

�RT

� �ð4Þ

where R is the universal gas constant, T is the absolute temperature, and 55.5 is the

concentration of water in bulk solution in M-1. The high value of K (Table 3)

reflects the high adsorption ability of these compounds on the Ni surface. The value

of K was found to be in the order: 1 [ 2 [ 3 [ 4 which runs parallel to the inhi-

bition efficiency.

Plotting C/h against C gives a straight line with an approximate unit slope value

(Fig. 2), indicating that the adsorption of drug compounds 1–4 on the nickel surface

follows the Langmuir adsorption isotherm and, hence, there is no interaction

between the adsorbed species. This deviation from unity is due to the Langmuir

isotherm, originally derived for the adsorption of gas molecules on solid surfaces,

which was modified to fit the adsorption isotherm of solutes onto solid surfaces in

Table 2 The effect of inhibitor concentration on the free corrosion potential (Ecorr), corrosion current

density (icorr), Tafel slopes (ba and bc), inhibition efficiency (% IE), degree of surface coverage (h), and

polarization resistance (Rp) for the corrosion of nickel in 1 M HCl at 25 �C

Comp. Conc.

(ppm)

-Ecorr (mV)

vs. SCE

icorr (lA

cm-2)

bc, (mV

dec-1)

ba (mV

dec-1)

Rp 9 10-2

(X cm2)

h % IE

1 0.0 311 15.91 265 282 3.726 – –

10 234 5.116 226 276 10.56 0.679 67.9

20 330 4.284 221 220 11.20 0.731 73.1

30 329 4.066 224 224 11.99 0.744 74.4

40 317 3.619 290 210 12.56 0.773 77.3

50 330 3.356 209 214 13.70 0.789 78.9

60 325 1.236 192 204 34.79 0.922 92.2

2 10 240 4.120 248 250 13.13 0.741 74.1

20 263 3.848 234 242 26.48 0.758 75.8

30 288 3.480 215 232 13.95 0.781 78.1

40 297 2.061 204 235 23.08 0.871 87.1

50 252 1.787 202 189 23.81 0.888 88.8

60 273 1.397 201 172 32.14 0.912 91.2

3 10 295 3.915 212 219 12.00 40.75 75.4

20 236 3.481 223 231 14.18 0.781 78.1

30 285 2.971 208 207 15.19 0.813 81.3

40 182 2.471 232 242 20.83 50.84 84.5

50 280 2.239 189 189 18.40 0.859 85.9

60 271 1.925 197 186 21.63 0.879 87.9

4 10 315 9.890 250 250 57.58 0.378 37.8

20 317 9.443 246 246 60.35 0.406 40.6

30 302 6.812 231 231 77.46 0.572 57.2

40 269 3.644 218 218 13.69 0.771 77.1

50 237 2.624 219 219 17.55 0.835 83.5

60 229 2.524 218 218 18.12 0.841 84.1


123

solution. A modified Langmuir adsorption isotherm [26] could be applied to this

phenomenon, which is given by the corrected equation:

C=h ¼ n=K þ nC ð5Þ

where n is the value of slopes obtained from the plot in Fig. 2. The aim of modifi-

cation was based on the fact that direct application of the Langmuir isotherm to

solution systems often leads to poor data fitting [27]. The negative value of DG�ads

(Table 3) indicates spontaneous adsorption of investigated compounds on the Ni

surface and also the strong interaction between inhibitor molecules and the metal

surface [28]. Generally, the standard free energy values of -20 kJ mol-1 or less

negative are associated with an electrostatic interaction between charged molecules

and the charged metal surface (physical adsorption), those of -40 kJ mol-1 or more

negative involves charge sharing or transfer from the inhibitor molecules to the metal

surface to form a co-ordinate covalent bond (chemical adsorption) [29]. The cal-

culated standard free energy of adsorption values is\10 kJ mol-1. Therefore, it can

be concluded that these compounds are physically adsorbed on the Ni surface [30].

Table 3 Inhibitor binding

constant (K), free energy of

binding (DG�ads), of the

investigated compounds for the

corrosion of nickel in 1 M HCl

at 25 �C

Inhibitors Langmuir adsorption isotherm

K 9 10-4 (M-1) -DGadso (kJ mol-1)

1 9.2 9.8

2 3.1 7.0

3 2.1 6.1

4 1.0 4.2

10 20 30 40 50 60

10

20

30

40

50

60

70

C/ θ

C,PPM

Compound(1) R2= 0.9969 Compound(2) R2= 0.9984 Compound(3) R2= 0.9984 Compound(4) R2= 0.9999

Fig. 2 Curve fitting of corrosion data obtained from potentiodynamic polarization method for nickel in1 M HCl in the presence of different concentrations of investigated compounds to the Langmuiradsorption isotherm at 25 �C


123

Effect of temperature

The importance of temperature variation in corrosion studies involving the use of

inhibitors is to determine the mode of inhibitor adsorption on the metal surface.

Recently, the use of two temperatures to establish the mode of inhibitor adsorption

on a metal surface has been reported and has been found to be adequate [31, 32].

Thus, the influence of temperature on the corrosion behavior of Ni in 1 M HCl in

the absence and presence of cephalosporin antibiotics of varying concentrations

were investigated by the potentiodynamic method at 25 and 40 �C. Therefore, in

examining the effect of temperature on the corrosion process, the apparent

activation energies (Ea*) were calculated from the Arrhenius equation [33]:

Log q2=q1

� �¼ E

�a=2:303R

1=T2 � 1=T1½ � ð6Þ

where q2 and q1 are the corrosion rates at temperature T2and T1, respectively, and

R is the universal gas constant.

Increased activation energy (Ea*) in inhibited solutions compared to the blank

suggests that the inhibitor is physically adsorbed on the corroding metal surface, while

either unchanged or lower Ea in the presence of inhibitor suggest chemisorptions [34].

It is seen from Table 4 that Ea values were higher in the presence of the additives

compared to those in their absence, hence leading to a reduction in the corrosion rates.

It has been suggested that adsorption of an organic inhibitor can affect the corrosion

rate by either decreasing the available reaction area (geometric blocking effect) or by

modifying the activation energy of the anodic or cathodic reactions occurring in the

inhibitor-free surface in the course of the inhibited corrosion process [35]. The Ea*

values support the earlier proposed physisorption mechanism. Hence, corrosion

inhibition is assumed to occur primarily through physical adsorption on the nickel

surface, giving rise to the deactivation of these surfaces to hydrogen atom

recombination. Similar results have been reported in earlier publications [36].

AC impedance technique

The corrosion behavior of nickel in 1 M HCl solution in the absence and presence of

different concentrations of the investigated compounds was investigated by the EIS

method at the open circuit potential conditions at 30 �C. Figure 3 shows the Nyquist

plots for nickel in 1 M HCl solution in the absence and presence of different

concentrations of compound 4 at 25 �C, respectively. Similar curves were obtained

Table 4 Activation energy of

the corrosion of nickel in

1 M HCl at 60 ppm investigated

compounds

Inhibitor Ea* (kJ mol-1)

Free acid 10.9

1 65.6

2 60.4

3 58.0

4 51.9


123

for other inhibitors (not shown). The Nyquist diagram obtained with 1 M HCl

shows only one capacitive loop, both in uninhibited and inhibited solutions, and the

diameter of the semicircle increases on increasing the inhibitor concentration

suggesting that the formed inhibitive film was strengthened by the addition of

inhibitors. The corresponding Bode plots are shown in Fig. 4 and all the main

parameters deduced from the impedance technique are given in Table 5. The

impedance data of nickel in 1 M HCl are analyzed in terms of an equivalent circuit

model Fig. 5 which includes the solution resistance Rs or RX and the double layer

capacitance Cdl which is placed in parallel to the charge transfer resistance Rct [37]

due to the charge transfer reaction.

Cdl ¼ 1 = 2pfmaxRctð Þ ð7Þ

where fmax is the maximum frequency. The inhibition efficiencies and the surface

coverage (h) obtained from the impedance measurements are defined by the fol-

lowing relations:

%IE ¼ 1� R�ct=Rct

� �� 100 ð8Þ

h ¼ 1� R�ct=Rct

� �� ð9Þ

where Rcto and Rct are the charge transfer resistance in the absence and presence of

inhibitor, respectively. From the impedance data given in Table 5, we conclude that

the value of Rct increases with increasing the concentration of the inhibitors, indi-

cating the decreased corrosion rate (i.e. increased corrosion inhibition) in acidic

solution. As the impedance diagram obtained has a semicircle appearance, it shows

that the corrosion of nickel is mainly controlled by a charge transfer process. The

value of double layer capacitance (Cdl) decreases by increasing the inhibitor con-

centration, indicating the reduction of charges accumulated in the double layer due

to the formation of adsorbed inhibitor layer [38], and its lower values indicate the

inhomogeneity of surface of the metal has been roughened due to corrosion. The

0 100 200 300 400 500 600 700 800 9000

-50

-100

-150

-200

-250

-300

-350

Zim

age O

hm c

m–2

Zreal

Ohm cm–2

Blank 30 ppm (4) 40 ppm (4) 50 ppm (4) 60 ppm (4)

Fig. 3 The Nyquist plots for nickel in 1 M HCl solution in the absence and presence of differentconcentrations of compound 4 at 25 �C


123

inhibition efficiencies calculated according to the impedance results are in the order:

1 [ 2 [ 3 [ 4, and these results follow the same trend as the polarization results.

The % IE obtained from EIS measurements are close to those deduced from

potentiodynamic polarization method.

-2 -1 0 1 2 3 4 5 6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

log

Z m

od(o

hm.c

m–2

)

log Freq (Hz)

Blank (1 M Hcl) 30 PPM (4) 40 PPM (4) 50 PPM (4) 60 PPM (4)

0

-20

-40

-60

-80

θ

Fig. 4 The Bode plots for nickel in 1 M HCl solution in the absence and presence of differentconcentrations of compound 4 at 25 �C

Table 5 Electrochemical kinetic parameters obtained by EIS technique for corrosion of nickel in 1 M

HCl at different concentrations of investigated compounds at 25 Æ C

Compound Conc. (ppm) Rct (X cm2) Cdl (lF cm-2) h % IE

Free acid 0 372.6 39.86 – –

1 30 1,146 27.56 0.675 67.5

40 1,181 20.02 0.685 68.5

50 1,518 18.6 0.755 75.5

60 2,833 16.49 0.869 86.9

2 30 880.1 33.99 0.577 57.7

40 1,053 29.57 0.646 64.6

50 1,329 23.67 0.732 73.2

60 1,534 19.02 0.757 75.7

3 30 610.4 37.59 0.389 38.9

40 713.5 31.42 0.478 47.8

50 920.0 28.20 0.595 59.5

60 1,067 31.42 0.651 65.1

4 30 487.4 34.26 0.236 23.6

40 537.2 32.57 0.306 30.6

50 655.1 32.88 0.431 43.1

60 706.3 31.78 0.472 47.2


123

Electrochemical frequency modulation (EFM) technique

The EFM technique is used to calculate the anodic and cathodic Tafel slopes as well

as corrosion current densities for the system Ni/HCl without and with various

concentrations of compound 4 at 25 �C. Figures 6, 7, 8, 9, and 10 are examples

representing the EFM intermodulation spectra (spectra of current response as a

function of frequency) of nickel in aerated 1 M HCl solutions. Similar results were

recorded for the other concentrations. The inhibition efficiency, % IE, of compound

4 was calculated at different concentrations using equation presented elsewhere

[39].

%IEEFM ¼ ½1� ðicorr=i�corrÞ� � 100 ð10Þ

where icorr and i8corr are the current densities in absence and presence of inhibitors,

respectively.

The calculated electrochemical parameters icorr, CF2, CF3, and % IE are given in

Table 6. Inspections of these data infer that the values of causality factors obtained

under different experimental conditions are approximately equal the theoretical

values (2) and (3) indicating that the measured data are of high quality [40]. In the

absence of the inhibitors (blank), the value of corrosion current density (icorr) can be

seen, and hence the rate of corrosion. Addition of increasing concentrations of

compound 4 to the HCl solution decreases the corrosion current density (icorr) at a

given temperature, indicating that compound 4 inhibits the acid corrosion of nickel

through adsorption. However, at a given inhibitor concentration, the corrosion

current density (icorr) still increases with increasing the temperature as a result of

Fig. 5 Equivalent circuit modelfits the impedance data

Fig. 6 Intermodulation spectrum recorded for nickel electrode in presence of 1 M HCl at 25 �C


123

increasing the rate of corrosion and partial adsorption of inhibitor species on the

nickel surface. The calculated inhibition efficiency, % IE enhances with compound

4 concentration. The inhibition efficiencies calculated according to the EFM results

are in the order: 1 [ 2 [ 3 [ 4, and these results follow the same trend as the

polarization and impedance results.

Mechanism of corrosion inhibition

It is generally assumed that adsorption at the metal/solution interface is the first step

in the inhibition mechanism in aggressive acidic media, which as most organic

Fig. 8 Intermodulation spectrum recorded for nickel electrode in 1 M HCl solution in presence of40 ppm of compound 4 25 �C


Fig. 7 Intermodulation spectrum recorded for nickel electrode in 1 M HCl solution in presence of30 ppm of compound 4 at 25 �C


123

compounds contain at least one polar group with an atom of nitrogen, sulfur or

oxygen, each might be a chemisorptions center. The inhibitive action depends on the

electron densities around the adsorption center; the higher the electron density at the

center, the more efficient is the inhibitor. Inhibition efficiency depends on several

factors such as the number of adsorption sites and their charge density, molecular

size, heat of hydrogenation, mode of interaction with the metal surface, and extent of

the formation of metallic complexes [41]. The order of inhibition efficiency obtained

from electrochemical measurements is as follows: 1 [ 2 [ 3 [ 4.

Table 6 Electrochemical kinetic parameters obtained by EFM technique recorded for nickel electrode in

1 M HCl with additives of various concentrations at 25 Æ C

Compound Conc. (ppm) icorr (lA cm-2) Causality factor (2) Causality factor (3) h % IE

Free acid 0.0 54.25 1.916 2.745 – –

1 30 17.96 1.967 3.053 0.669 66.9

40 13.02 2.026 3.282 0.760 76.0

50 10.56 1.891 3.324 0.805 80.5

60 7.94 2.003 2.881 0.854 85.4

2 30 24.28 1.871 2.797 0.552 55.2

40 18.27 1.880 3.053 0.663 66.3

50 16.30 1.948 3.151 0.700 70.0

60 12.08 1.786 2.853 0.777 77.7

3 30 33.75 1.957 2.797 0.378 37.8

40 24.28 1.871 3.053 0.552 55.2

50 22.36 2.290 3.151 0.588 58.8

60 18.60 1.902 2.853 0.657 65.7

4 30 40.52 1.918 3.240 0.253 25.3

40 36.21 1.804 2.883 0.333 33.3

50 33.56 1.942 3.372 0.381 38.1

60 29.74 1.957 3.352 0.452 45.2



123

The adsorption of these inhibitors at the Ni surface can take place through their

active centers, N, O and S atoms, in addition to a p electron interaction of the

benzene ring nucleus with unshared d electrons of Ni atoms [42–45]. The adsorption

and the inhibition effect of investigated inhibitors in 1 M HCl solution can be

explained as follows: inhibitor molecules might be protonated in the acid solution

(compound 1 as example) as:

C17H19N3O3S½ � þ xHþ ! C17H19þ xN3O3S½ �xþ ð11ÞIn aqueous acidic solutions, these inhibitors exist either as neutral molecules or as

protonated molecules (cations). These inhibitors may adsorb on the metal/acid

solution interface [46] by one and/or more of the following ways: (1) electrostatic

attraction between charged molecules and charged metal, (2) interaction of unshared

electron pairs in the molecule with the metal, (3) interaction of p electrons with the

metal, and (4) a combination of the previous three.

In general, two modes of adsorption are considered on the metal surface in acid

media. In one mode, the neutral molecules may be adsorbed on the surface of the Ni

via a chemisorption mechanism, involving the displacement of water molecules

from the nickel surface and the sharing of electrons between the hetero-atoms and

the nickel. The inhibitor molecules can also adsorb on the Ni surface on the basis of

donor–acceptor interactions between p-electrons of the aromatic ring and vacant

d-orbitals of surface nickel atom. In the second mode, since it is well known that the

nickel surface bears a positive charge in acid solution [47], it is difficult for

the protonated molecules to approach the positively charged nickel surface due to

the electrostatic repulsion. Since chloride ions have a smaller degree of hydration,

so they could bring excess negative charges in the vicinity of the interface and favor

more adsorption of the positively charged inhibitor molecules, with the protonated

inhibitors adsorbing via electrostatic interactions between the positively charged

molecules and negatively charged metal surface. Thus, there is a synergism between

adsorbed Cl- ions and protonated inhibitors, and we can also conclude that

inhibition of nickel corrosion in 1 M HCl is mainly due to electrostatic interaction.

The decrease in inhibition efficiency with a rise in temperature supports electrostatic

interaction.

In organic compounds differing in the functional donor atom (other factors being

equal), the order of corrosion inhibition is usually: S [ N [ O, which is the reverse

order of electronegativity. Sulfur compounds are better corrosion inhibitors than

their nitrogen analogues because the S-atom, being less electronegative than N,

draws fewer electrons to itself, and is thus the more efficient electron donor in

forming the chemisorptive bond.

Compound 1 is the most efficient one, which is due to the presence of 3 S, 8 N,

and 7 O atoms in its structure, but compound 2 comes after compound 1 in

inhibition efficiency. This is due to the smaller number of oxygen atoms (4 O atoms)

in its structure. Compound 3 comes after compound 2 in inhibition efficiency. This

is due to the smaller number of nitrogen atoms (6 N atoms) and sulfur atoms (2 S

atoms) in its structure. Compound 4 is the least effective inhibitor. This is due to the

smaller number of nitrogen atoms (5 N atoms) in its structure.


123

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Cephalosporin antibiotics as new corrosion inhibitors for nickel in HCl solution

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